Systems of large lightly reinforced walls

Eurocode 8 is unique among international codes in that it includes special provisions for systems consisting of a fairly large number of large but lightly reinforced concrete walls which are designed to sustain seismic demands not by dissipating kinetic energy through hysteresis in plastic hinges but by converting part of this energy into potential energy of the masses and returning part to the ground through radiation from their foundation. To qualify for the special design provisions of Eurocode 8 such a system should have in each horizontal direction at least two walls (for redundancy and torsional resistance) that qualify as 'large lightly reinforced walls' in the sense of Section 5.2.4, resist together at least 65% of the seismic base shear in the horizontal direction of interest (for the system to qualify as a wall system) and support together at least 20% of the total gravity load (i.e. at least 40% in total for the walls of the two directions). The building should also have a fundamental period in each horizontal direction for assumed fixity of all vertical elements at the base against rotation of not longer than 0.5 s. This last condition promotes walls with a low aspect ratio and/or a large total cross-sectional area as a percentage of the total plan area of the floors, and takes into account better the effect of openings in the wall than a mere geometrical criterion would have done. The condition of at least 20% of the total gravity load carried by the walls of each horizontal direction ensures that the rocking motion of these walls increases the potential energy of at least that part of the total mass of the building. The condition of at least two large walls per horizontal direction may be relaxed, provided that (1) the two other conditions - for at least of 20% of total gravity load and for a period not more than 0.5 s - can be met with a single large wall in that direction, (2) there are at least two large walls in the orthogonal direction and (3) the q factor in the direction with just one large wall is reduced by one-third.

If the structural system meets all the conditions above, Section 5 permits all the walls that qualify as large to be designed and detailed in a very economic way according to the special rules for large lightly reinforced walls outlined in Section 5.8 below. The system of large lightly reinforced walls is considered to qualify for a basic q factor equal to that for wall systems with uncoupled ductile walls designed and detailed according to the much more demanding rules for ductile walls of Ductility Class Medium (DCM). Walls with length /w of less than 4 m (or two-thirds of the total height in buildings less than 6 m tall) in a system of large lightly reinforced walls should be designed and detailed according to the rules for ductile walls of DCM. These latter rules should also be followed by any wall with length lw over 4 m (or two-thirds of the total height in buildings less than 6 m tall), if in the direction of /w the system does not qualify as a system of large lightly reinforced walls.

Greener Homes for You

Greener Homes for You

Get All The Support And Guidance You Need To Be A Success At Living Green. This Book Is One Of The Most Valuable Resources In The World When It Comes To Great Tips on Buying, Designing and Building an Eco-friendly Home.

Get My Free Ebook


Post a comment